The present invention relates to a base station for mobile communication systems, such as Personal Handyphone System (hereinafter referred to as xe2x80x9cPHSxe2x80x9d), in which radio communication is performed between the base station and mobile stations in the service area by a diversity method.
In recent years, services in radio communication between a base station and a plurality of mobile stations have spread widely, and the PHS, which provides such communication services at lower rates, has been put to practice. In the PHS, an apparatus at a base station has become smaller and less expensive by reducing its transmission power. As a result, the service area of the base station has also become smaller, and a large number of base stations are spaced at shorter intervals.
FIG. 1 shows an example TDMA/TDD (Time Division Multiple Access/Time Division Duplex) frame (hereinafter referred to as xe2x80x9cTDD framexe2x80x9d) for communication between a base station CS and mobile stations PS1 to PS3 in the conventional PHS. In this figure, xe2x80x9c∩xe2x80x9d indicates a time slot, xe2x80x9cTxe2x80x9d indicates transmission, and xe2x80x9cRxe2x80x9d indicates reception. Under the PHS standard (RCR STD-28), each TDD frame should be 5 msec, and each time slot should be 625 xcexcsec.
In the conventional PHS, a time slot 1 (#1) in the TDD frame is used as a control channel, while time slots 2 to 4 (#2 to #4) are used as communication channels.
The base station CS uses the control channel #1 (#1T and #1R) to perform communication every 100 msec for registering the position of the mobile stations PS within the service area and for setting communication channels for the mobile stations PS. For instance, the base station CS and each mobile station PS communicate with each other on a carrier frequency of the carrier number 71 (1916.150 MHz) prescribed in the PHS standard.
In the above communication channel setting process, the base station CS allocates one vacant slot of the communication channels (#2 to #4) to a corresponding mobile station PS. A carrier frequency selected from available outdoor public communication carriers prescribed by the PHS standards is allocated to the mobile station PS for communication in the allocated slot. The mobile station PS then switches to the allocated communication channel and communicates in the allocated slot on the allocated frequency.
When the base station CS conducts radio communication with the mobile station PS in the above PHS, there is a problem that communication between the base station CS and the mobile station PS deteriorates due to fading and the like. Fading is caused by interference between an electric wave which has directly reached the mobile station from the base station and a reflected wave or diffracted wave which has reached the mobile station after being reflected by a building or the like. For this reason, communication systems such as reception diversity and transmission diversity have been employed.
Diversity methods are aimed at reducing adverse influence of fading by composing the outputs of two or more systems which have little correlation with each other and little possibility of lowering their communication quality at the same time. Reception diversity methods include a method in which a plurality of antennas arranged at predetermined intervals receive electric waves, and the input received by the antenna that gives the highest input signal level (or RSSI: Received Signal Strength Indicator) is selectively demodulated. Transmission diversity methods include a method in which a plurality of antennas arranged at predetermined intervals receive electric waves, and transmission is performed through the antenna that gives the highest input signal level.
As described above, conventional PHS base stations have been made compact and the production costs have been lowered by reducing the transmission power of each base station, so that subscribers can get the communication service at low rates.
If many base stations are installed in an expensive area, such as an urban area or a central area in the national capital region, the installation costs of the base stations will be higher even though the base stations themselves are inexpensive. For this reason, there have been demands that the base stations be installed at longer intervals, while allowing a reasonable rise in price due to the improved functions and high performance.
In an urban area or a central area in the national capital region, the communication traffic per unit area is high. If the installation density of base stations in such areas, the quality of the communication service provided will be lower, leaving the above problems unsolved.
To solve the above two problems, that is, to reduce the installation density and maintain the quality of communication service, each base station should have higher transmission and reception performance, and be capable of connecting more mobile stations per unit time.
More specifically, the following method can be employed. One base station should be able to relay communication data to more mobile stations using a plurality of TDD frames simultaneously. In the TDD frames, different carrier frequencies are allocated to corresponding time slots.
FIG. 2 shows channel allocation of each TDD frame on the base station side, where different carrier frequencies are allocated to corresponding time slots of two TDD frames. In this figure, xe2x80x9c#xe2x80x9d indicates each time slot, xe2x80x9cTxe2x80x9d indicates transmission, and xe2x80x9cRxe2x80x9d indicates reception.
A first TDD frame shown in FIG. 2 consists of time slots 1 to 4 (#1 to #4) in both the upstream and downstream, like the TDD frame shown in FIG. 1. The time slot #1 is used as a control channel, and communication in the control channel is conducted on a carrier frequency of 1916.150 MHz (carrier number 71). The time slots #2 to #4 are used as communication channels.
A second TDD frame shown in FIG. 2 is in synchronization with the first TDD frame, and consists of time slots 5 to 8 (#5 to #8) both in the upstream and downstream. The time slots #5 to #8 can be all used as communication channels, because the time slot #1 of the TDD frame is allocated as the control channel.
As for the carrier frequencies, a carrier frequency allocated to the communication channel #5 is naturally different from the carrier frequency allocated to the control channel #1. As for two corresponding communication channels used at the same time, two different carrier frequencies of the outdoor public communication frequencies, other than the frequency of carrier number 71, are allocated. For instance, two different carrier frequencies are allocated to the time slot #2 of the first TDD frame and the time slot #6 of the second TDD frame.
By using two TDD frames as above, one base station can relay communication data to more mobile stations than in the case where two base stations CS of FIG. 1 are used for communication. The number of mobile stations in this case is one larger than in the case where two base stations CS are used, that is, the two TDD frames enable one base station to relay communication data to seven mobile stations.
FIG. 3 is a block diagram showing the main part of a PHS base station 500 for communicating with mobile stations using the two TDD frames shown in FIG. 2.
The base station 500 includes antennas 501 to 504, transmit-receive selecting switches 511 to 514, receiving units 521 to 524, a judging unit 531, a selecting unit 532, HPAs (High Power Amplifiers) 541 and 542, antenna selecting switches 551 and 552, and composition units 561 to 564.
The transmit-receive selecting switches 511 to 514 switch the respective antennas 501 to 504 between transmission and reception in both downstream and upstream time slots in the TDD frames.
The receiving units 521 to 524 have a uniform structure. From input signals received by the antennas 501 to 504, the receiving units 521 to 524 extract modulation signals of two systems having different carrier frequencies allocated to each time slot. The receiving units 521 to 524 also measures the input signal level of each extracted modulation signal, and then output the measured values to the judging unit 531.
The judging unit 531 judges which measured value shows the highest input signal level among the input signal level values of the system 1 measured by the receiving units 521 to 524, and also judges which antenna has given the highest input signal level. The judging unit 531 then outputs the judgement result to the selecting unit 532. At the same time, the judging unit 531 judges which measured value shows the highest input signal level among the input signal level values of the system 2 measured by the receiving units 521 to 524, and also judges which antenna has given the highest input signal level. It then outputs the judgement result to the selecting unit 532.
The selecting unit 532 outputs select signals to the antenna selecting switches 551 and 552 so as to select one of the composition units 561 to 564. The selected composition unit should correspond to the antenna whose highest input signal level determined by the judging unit 531 is higher than the highest input signal levels of other antennas. Here, the two antennas have been determined for each downstream time slot by the judging unit 531 in the previous upstream (reception) time slot in the TDD frames.
The HPA 541 outputs the modulation signal of the system 1 to be transmitted to a mobile station allocated to each time slot of the first TDD frame to the antenna selecting switch 551. The modulation signal has been amplified by the HPA 541 before being outputted.
The HPA 542 outputs the modulation signal of the system 2 to be transmitted to a mobile station allocated to each time slot of the second TDD frame to the transmit-receive selecting switch 552. The modulation signal has been amplified by the HPA 542 before being outputted.
The antenna selecting switch 551 selects one of the four output terminals connected to the composition units 561 to 564 in accordance with the select signals from the selecting unit 532, and then outputs the amplified modulation signal of the system 1 via the selected output terminal.
The antenna selecting switch 552 selects one of the four output terminals connected to the composition units 561 to 564 in accordance with the select signals from the selecting unit 532, and then outputs the amplified modulation signal of the system 2 via the selected output terminal. Thus, the output terminal to be connected to the same composition unit is selected by the antenna selecting switches 551 and 552 in accordance with the select signals from the selecting unit 532.
The composition units 561 to 564 have a uniform structure. When the modulation signals of both system 1 and system 2 are inputted, each of the composition units 561 to 564 combines the two inputs, and then outputs the combined input to the corresponding transmit-receive selecting switch in accordance with the select signals from the selecting unit 532.
The mobile stations communicate with the base station 500 on a carrier frequency of 1916.150 MHz (carrier number 71) using the control channel allocated to #1 slot of the TDD frame so as to register their positions in the waiting state. After the base station 500 allocates each mobile station a communication channel and carrier frequency, each mobile station sets its reception and transmission frequency at the allocated frequency, and performs communication using the allocated communication channel.
With this structure, the base station 500 can relay communication data to up to seven mobile stations within the service area. This base station 500 relays high-quality communication data to the mobile stations by performing reception diversity and transmission diversity using four antennas. Since the base station 500 can relay communication data without adding new elements to the structure of the mobile stations, conventional mobile stations can be used in the PHS.
Since the composition units 561 to 564 have a uniform structure, only the composition unit 561 is described below.
FIG. 4A shows the structure of the composition unit 561. In this figure, the input side of the composition unit 561 is shown on the left side. The composition unit 561 takes the form of branch lines as shown in FIG. 4A. One terminal on the output side in this form is grounded via a terminal resistance of 50xcexa9 which is impedance-matched in the transport path of the modulation signal of each system. The other terminal on the output side in this form is connected to the transmit-receive selecting switch 511.
If the antenna selecting switches 551 and 552 select the composition unit 561, the modulation signal amplified by the HPA 541 (carrier frequency f1) and the modulation signal amplified by the HPA 542 (carrier frequency f2) are inputted to the input terminals corresponding to the antenna selecting switches 551 and 552, respectively.
The outputs of the HPAs 541 and 542 reach the antennas after passing through the antenna selecting switches, the composition units, and the transmit-receive selecting switches. The antenna selecting switches and the transmit-receive selecting switches give resistance to each transport path from the HPAs to the antennas, and the transmission power loss caused by the resistance is 1 dB to 1.5 dB in total in each system. The transmission power loss in each composition unit is even greater. More specifically, as can be seen from the pattern shown in FIG. 4A, input power is divided into the output terminals and grounded terminal in the composition unit 561, and 50% of power is lost on the output side. FIG. 4B shows the power loss in the composition unit 561. In this figure, the axis of abscissas indicates the carrier frequencies of the modulation signals inputted into the composition unit 561, while the axis of ordinate indicates the power ratio of the composition unit 561. As shown in FIG. 4B, the power loss in the composition unit 561 is 3 dB.
As described above, the conventional base station 500 has the problem that a transmission power loss of 4 to 4.5 dB in power ratio, including the loss due to the composition units, switches, and wires, is caused in the transport path from the HPA 541 or 542 to the antennas 501 to 504.
To increase the transmission power of the base station 500 up to 500 mW to meet such a great power loss, the outputs of the HPAs 541 and 542 need to be set twice as large. Since the HPAs are made up of expensive devices having excellent high-frequency characteristics, such as GaAs FET, the production of the base station 500 is costly. The HPAs need to include amplifiers with large radio power.
Each of the composition units 561 to 564 is provided with a HPA in its later stage so as to compensate for the loss caused by the composition units 561 to 564. In such a case, the outputs of the composition units 561 to 564 should be collectively amplified by one linear amplifier, and therefore, an expensive linear amplifier which can amplify modulation signals while keeping excellent linearity over a wide range is necessary, causing the problem that the production of the base station 500 is costly.
Since the high-frequency outputs of the two systems are allocated to one of the four antennas, the circuit structure of the radio system (high-frequency circuits) is complicated, and the wiring is very difficult, because a high-frequency circuit often causes wire coupling, which results in spurious. For those reasons, it has been difficult to make high-frequency circuits compact.
The object of the present invention is to provide a base station for mobile communication systems which can perform a large quantity of simultaneous connection while restraining the rise of cost with a simple structure, and keeping high communication quality.
The other object of the present invention is to provide a base station for mobile communication systems whose wire arrangement is simple, and makes it possible to minimize the circuits, which also reduces the production costs.
To achieve the above objects, the base station of the present invention comprises a plurality of generating units for generating high-frequency transmission signals, a plurality of antennas, the number of which is larger than the number of the generating units, a switching unit for switching connection of each output terminal of the plurality of generating units from one antenna to another, and a control unit for controlling the switching unit so that the output terminals of the plurality of generating units will be connected to different antennas.
With this structure, the transmission antenna switching can be performed by the switching unit. Since there is no need to provide composition units which are necessary in the prior art, inexpensive devices can be used for the generating units (amplifying units) for outputting transmission signals, reducing the production costs.
The above base station may further comprise a measuring unit for measuring the input signal level of each antenna, a level judging unit for judging which antenna has the highest input signal level among all the antennas for each of the input signals having different frequencies, and an allocating unit for allocating the highest level antenna to a transmission signal corresponding to each input signal.
The control unit may comprise an overlap judging unit for judging whether the antenna selected by the level judging unit is repeatedly allocated to different high-frequency signals, a re-allocating unit for allocating the high-frequency signal having the highest input signal level to an antenna other than the selected antenna in the case where the selected antenna is allocated to two or more high-frequency signals, and a switch control unit for controlling the switching unit in accordance with allocation and re-allocation results.
With this structure, a carrier wave whose highest input signal level is the lowest is allocated to the first antenna, while two carrier waves are prohibited from being transmitted through one antenna. Thus, the transmission diversity can be performed most effectively.
The base station may further comprise a plurality of data output units for outputting transmission data, and an interchanging unit for inputting transmission data from the plurality of data output units interchangeably to the plurality of generating units. Here, the number of the data output units provided is the same as the number of the generating units.
The control unit may collectively control the switching unit and the interchanging unit so that the output terminals of the generating units will be connected to different antennas.
With this structure, the antenna switching is performed using the switching unit for switching high-frequency transmission signals and the interchanging unit for interchanging transmission data at the stage of low-frequency signal. Since there is no need to provide composition units which are necessary in the prior art, high-frequency power loss can be minimized. Thus, inexpensive devices can be used for the generating units (amplifying units) for outputting transmission signals, and the production costs can be reduced. Furthermore, since the number of wires for switching antennas on a high frequency can be reduced by interchanging low-frequency signals, the wire arrangement of the high-frequency circuits can be simpler, and the circuits themselves are smaller.
Each of the generating units may include a first and second generating units.
The switching unit may be provided with a first switching unit for switching connection of the output terminal of the first generating unit from one antenna to another among a predetermined group in the plurality of antennas, and a second switching unit for switching connection of the output terminal of the second generating unit from one antenna to another among the remaining antennas.
With this structure, since the plurality of antennas are divided into two groups corresponding to the first and second generating units, the number of wires on a high frequency can be reduced further. Here, circuit coupling on a high frequency can also be reduced, and the design and production of the circuits can be easier and smaller.
The switching unit may further comprise a third switching unit for bypass-connecting the output terminal of the second generating unit to one of the antennas on the first switching unit side, and the output terminal of the first generating unit to one of the antennas on the second switching unit side.
With this structure, although the antennas are divided into two groups, the number of antennas to be selected for each transmission signal by bypass connection will not be reduced.
The first and second generating units each may include a PLL unit for generating local frequency signals to determine the frequency of each transmission signal. The control unit may control the local frequencies of the PLL unit so as to interchange the carrier frequencies of the first and second generating units when cross-connecting the interchanging unit.
With this structure, it is easy to interchange transmission data as well as carrier frequencies.
The base station of the present invention is also used in a mobile communication system performing diversity as well as transmission and reception by time-division bidirectional multiplex using synchronized time-division frames on two carrier waves. Here, the base station comprises four antennas, first and second generating units for generating high-frequency transmission signals each having a PLL unit for generating local frequency signals to determine the carrier frequency of each transmission signal, a first switching unit for switching connection of the output terminal of the first generating unit between predetermined two of the four antennas, a second switching unit for switching connection of the output terminal of the second generating unit between the remaining two antennas, a third switching unit for bypass-connecting the output terminal of the second generating unit to one of the antennas on the first switching unit side and the output terminal of the first generating unit to one of the antennas on the second switching unit side, first and second data output units for outputting transmission data, an interchanging unit for inputting the transmission data from the first and second data output units interchangeably to the first and second generating units, and a control unit for controlling the first, second, and third switching units, and the interchanging unit so that the output terminals of the generating units will be connected to different antennas, and for controlling the PLL unit so as to interchange the carrier frequencies of the first and second generating units when cross-connecting the interchanging unit.
With this structure, the antenna switching is performed using the switching unit for switching high-frequency transmission signals and the interchanging unit for interchanging transmission data in the stage of low-frequency signals. Since there is no need to provide composition units which are necessary in the prior art, high-frequency power loss can be reduced. Thus, inexpensive devices can be used for the generating units (amplifying units) for outputting transmission signals, reducing production costs. Since circuit coupling on a high frequency can also be reduced, the design and production of the circuits can be easier and smaller.
The first switching unit may include a first selecting switch having an input terminal to which the output terminal of the first generating unit is connected, the same number of output terminals as the antennas of the predetermined group, and a bypass output terminal, and transmit-receive selecting circulators for connecting the output terminals of the first selecting switch to the antennas of the predetermined group. The second switching unit may include a second selecting switch having an input terminal to which the output terminal of the second generating unit is connected, the same number of output terminals as the remaining antennas, and a bypass output terminal, and transmit-receive selecting circulators for connecting the output terminals of the second selecting switch to the remaining antennas.
The third switching unit may include a first bypass line for connecting the bypass output terminal of the first selecting switch to one of the circulators of the second switching unit, and a second bypass line for connecting the bypass output terminals of the second selecting switch to one of the circulators of the first switching unit.
The control unit may control bypass connection by open- or short-circuiting one port of each circulator to which the first and second bypass lines are connected so as to cause power total reflection.
With this structure, switching and bypassing can be performed on a high frequency using circulators which are passive devices causing little power loss.
The base station further comprises a measuring unit for measuring the level of an input signal corresponding to each transmission signal for each antenna, and level judging unit for judging which antenna has the highest input signal level for each carrier wave.
The control unit may include an allocating unit for allocating an antenna which has been judged to have the highest input signal level for each carrier wave, an overlap judging unit for judging whether the same antenna is allocated to different carrier waves, a re-allocating unit for allocating another antenna to the carrier wave that has the highest input signal level in the case where the same antenna is allocated to different carrier waves, and a switch control unit for controlling the switching unit in accordance with allocation and re-allocation results.
With this structure, a carrier wave whose highest input signal level is the lowest is allocated to the first antenna, while two carrier waves are prohibited from being transmitted through one antenna. Thus, the transmission diversity can be performed most effectively.